Mars Global Surveyor
Mars Orbiter Camera

Evidence for Recent Liquid Water on Mars:
Basic Features of Martian Gullies

Water is the chief agent of weathering and erosion on Earth. Mars is
a much drier, colder planet on which liquid water cannot exist very
long at the surface because it will immediately begin to boil,
evaporate, and freeze--all at the same time. However, new pictures
from the Mars Orbiter Camera (MOC) onboard the Mars Global Surveyor
(MGS) have provided an astonishing observation which suggests that
liquid water may have played a role in shaping some recent gully-like
features found on the slopes of various craters, troughs, and other
depressions on the red planet.

These pictures introduce the basic features of a martian gully. The
figure on the left is an example from Mars, the figure on the right is
a gully on Earth. In the Earth picture, rain water flowing under and
seeping along the
base of a recently-deposited volcanic ash layer has created the gully.
For Mars, water is not actually seen but is inferred from the
landforms and their similarity to examples on Earth.

The landforms both on Earth and Mars are divided into three parts: the
alcove, the channel, and the apron. Water seeps
from between layers of rock on the wall of a cliff, crater, or other
type of depression. The alcove forms above the site of seepage as
water comes out of the ground and undermines the material from which
it is seeping. The erosion of material at the site of seepage causes
rock and debris on the slope above this area to collapse and slide
downhill, creating the alcove.

The channel forms from water and debris running down the slope from the
seepage area. The point where the top of the channel meets the bottom
of the alcove is, in many cases, the site where seepage is occurring.
Channels are probably flushed-clean of debris from time to time by
large flash floods of water released from behind an ice barrier that
might form at the site of seepage during more quiescent times.

The aprons are the down-slope deposits of ice and debris that were
moved down the slope and through the channel. Whether any
water--likely in the form of ice--persists in these deposits is
unknown. The fact that the aprons do not go very far out onto the
floors of craters and troughs (e.g., the foreground of the
figure on the left) indicates that there is a limit as to how much
water actually makes it to the bottom of the slope in liquid form.
Most of the water by the time it reaches the bottom of the slope has
probably either evaporated or frozen.

The MOC image on the left was acquired July 3, 1999, and is located on
the south-facing wall of an impact crater near 54.8°S,
342.5°W. The MOC image is illuminated from the upper left; north
is toward the upper right. The MOC image covers an area 1.3 km (0.8
mi) wide by 2 km (1.2 mi) long.
The pictures from the flank of the Mount St. Helens volcano in
Washington (right; large image and inset) were taken by MGS MOC
Principal Investigator, Michael C. Malin, in the 1980s after the
eruptions of May 1980. They are illuminated from the left; note
footprints on left side of the picture for scale, also note the
colored bar, which is 30 cm (11.8 in) long.

Images Credit: NASA/JPL/Malin Space Science Systems

Click here to download a 3.2 MByte JPEG of the colorized
gully image without annotation.

Malin Space Science Systems and the California Institute of
Technology built the MOC using spare hardware from the Mars Observer
mission. MSSS operates the camera from its facilities in San Diego,
CA. The Jet Propulsion Laboratory's Mars Surveyor Operations Project
operates the Mars Global Surveyor spacecraft with its industrial
partner, Lockheed Martin Astronautics, from facilities in Pasadena, CA
and Denver, CO.